The invention relates to a device for detecting properties of a moving web of paper, more specifically for production control in the paper making process, the device having an infrared lighting device comprising an infrared light source and a light exit portion through which light emitted by the light source exits onto the paper web, having a detector device for detecting infrared light reflected or transmitted by the web of paper which is provided with a light input portion for guiding the reflected or transmitted light to a detector of the detector device, and having a housing a) which is arranged in immediate proximity to the paper web, b) which is provided with a front face facing the web of paper and c) which accommodates the light exit portion and/or the light input portion provided with optical fibers.
A device as described above is disclosed in WO 98/40727. In this known device, the web of paper is illuminated by an appropriate source of infrared light. The light input portion is located within the housing, which is substantially configured as a holding member. The light input portion is formed by a plurality of optical waveguides that are arranged side by side in the form of optical fibers of typically between 50 and 600 micrometers thick. The ends of the fibers are polished and oriented to face the web of paper, and are fastened to the housing and project therefrom toward the web of paper.
In principle, this arrangement has proved to be successful. The optical conditions however are not clear, which is disadvantageous. More specifically, the size of the measuring dot on the web of paper is difficult to control and, furthermore, does not apply for many measurement tasks. Furthermore, considerable amounts of dirt, dust and the like are produced in the paper making process. Cleaning the light input portion, more specifically the input ends of the optical fibers, involves difficulties.
Preferably, however, the advantages of the prior art device are to be maintained. The prior art device allows for a high number of measuring dots arranged side by side on the web of paper. Unlike cradles traveling back and forth on crossbars, the device has the advantage that light detection is carried out in a stationary condition relative to the crossbar. The mechanical expenditure needed for performing a reciprocating motion is not required. Moreover, the untested regions of the paper web are smaller than with a cradle traveling back and forth and carrying an optical detector.
In view of the drawbacks of the device of the type mentioned above, the object of the invention is to allow for a better optical allocation of the light exit portion and/or of the light input portion to the actual measuring dots and to concurrently improve the device in such a manner that it can be readily cleaned when dirty.
The solution to this object is to arrange a plurality of n spherical lenses in a sealed manner in the front face of the device housing, the spherical lenses being each optically coupled to an allocated optical fiber within the housing, the spherical lenses constituting, together with the corresponding optical fibers, the light exit portion and/or the light input portion of the device.
Accordingly, the invention provides the housing with exit windows. These exit windows are configured as spherical lenses, i.e., they also have optical properties. These properties permit to selectively alter the size of the image dot on the web of paper; spherical lenses of various diameters can be used or other optical provisions can be made for this purpose. The spherical lenses are well suited as exit windows; with simple means they can be arranged in a sealed manner in the front face, which is in most cases configured as a front plate. As they are spherical, they need not be oriented in any particular manner and resist high changes in temperature. Similarly, sealing is substantially carried out on a spherical annular region of the spherical lens. On thermal expansion of the materials of the front face and of the spherical lens, the spherical lens can move across the front face without being clamped. Accordingly, it is particularly advantageous to have the spherical lens sealed at some distance from its equator so that the major part of the spherical lenses is located within the housing and only a small part of the sphere protrudes from the front face of the housing toward the web of paper.
The aperture of the optical fibers is corrected or the light is collimated respectively by means of the spherical lenses. The size of the measuring dot on the web of paper can thus be varied; the diameter for example can be varied between 5 mm and 10 mm. In this way, paper can be measured with a sufficient cross sectional resolution of the profile measurement. Depending on the distance from the web of paper, which typically amounts to between 20 and 50 mm, on the size of the measuring dot and on the cross sectional resolution, spheres made of sapphire or of other glasses, which are transmissive to spectroscopy and have a diameter typically ranging from 2 to 10 mm, are used. They are simpler to manufacture than the customary lens-shaped convergent lenses and can be more readily fastened and sealed than the latter.
By virtue of the spherical lenses, the optical images on the paper web can be predetermined and controlled with much more precision. According to the invention in a first embodiment, all of the n spherical lenses are used for the light exit portion; in a second embodiment all of the lenses are used for the light input portion. In a third embodiment a first half n2 is used for the light input portion and the second half n2 for the light exit portion. More particularly, the first half n2 of spherical lenses is connected to optical fibers that lead to a source of infrared light of the infrared lighting device. A second half n2 is connected to respective optical fibers leading to a detecting part, more specifically a polychromator, of the detector device. The optical arrangements are selected such that, with the spherical lenses of the first half, substantially circular illuminated dots are obtained on the web of paper and that, with the spherical lenses of the second half, receiving dots (measuring dots) are obtained that are substantially circular as well, the receiving dots being smaller than the illuminated dots, though. Furthermore, the receiving dots are located within the illuminated dots. It is thus made certain that an optical signal is only acquired from the illuminated regions.
Within the housing, the spherical lenses are optically coupled to optical fibers. The housing is preferably sealed. The coupling region is protected as a result thereof.
Coupling two optical fibers by means of a spherical lens is known. For example, International Wire and Cable Symposium Proc. 1981, p. 341 describes coupling of two optical waveguides by means of spheres. The use of spherical lenses as an exit window and as a front end of a light exit portion or of a light input portion however is not known.
In a preferred embodiment, the spherical lenses are replaceably arranged in the front face so that spherical lenses of different diameters can be mounted into the front face. Different images can thus be obtained depending on the purpose of the measurement. The spherical lenses are preferably sapphire spheres, which are available at low cost on the market and have a high degree of hardness. Therefore, they can be readily cleaned without having to fear scratches.
Preferably, the plurality of n spherical lenses is accommodated in regular arrangement in the front face. The regular arrangement permits to assign with precision the illuminated dot formed by the light exit portion on the paper web and the corresponding receiving dot detected by the corresponding light input portion.
In another preferred embodiment, the plurality of n spherical lenses is connected to optical fibers leading to a detecting part, more specifically a polychromator, of the detector device and an infrared light source in the form of an elongated, linear radiating element is provided in the housing, the radiating element being allocated an exit window in the housing, more specifically an elongated cylindrical lens in the form of a rod which substantially is of the same length as the radiating element and is mounted into the front face so as to be sealed. The optical arrangements are thereby such that an elongated, narrow illuminated dot is projected on the web of paper while the n spherical lenses provide substantially circular receiving dots, the receiving dots being located side by side inside the elongated illuminated dot. Again, the optical conditions are predetermined in such a manner that only such optical signals are received and interpreted later on that originate from illuminated regions of the web of paper.
Although the device in accordance with the invention is in principle suited for light of any wavelength, it is preferably designed for the near infrared, by which the wavelength range above the visible range (above approximately 800 nm), more specifically 1.5 to 2.5 xcexcm, is meant.
In a particularly preferred embodiment there is provided a cleaning device which is designed to clean those portions of the spherical lenses and their surroundings that are accessible from the outside on the front face and that are oriented toward the web of paper. It is thus made certain that dirt, which cannot be excluded, can be removed and that the optical conditions can repeatedly be restored to their initial condition even when the paper making process is continuous and lasts for many months.
Preferably, the housing is configured in a substantially elongated shape, more specifically to give it a length of at least one meter. The usual widths of paper webs require several such housings that are arranged side by side in a staggered pattern on a stationary crossbar. To permit the exchange of individual housings during the paper making process, at least some of the housings are connected to the crossbar by way of a longitudinal guide along which they are slidable relative to the crossbar. Individual housings can thus be removed from the crossbar without disturbing the paper making process.